1. Field of the Invention
The present invention relates to a process for preparation of 5-pyrazolemercaptan derivatives useful for synthesis of sulfonylurea-based herbicides. More particularly, the present invention relates to a process for preparation of 5-pyrazolemercaptan derivatives represented by the following general formula (I): ##STR2## wherein R.sup.1 represents hydrogen, C.sub.1 -C.sub.4 alkyl, allyl or propargyl, R.sup.2 represents hydrogen, C.sub.1 -C.sub.4 alkyl, or a phenyl group which can contain one or more substituent selected from the group consisting of halogen, nitro and methyl at an optional position, and R.sup.3 represents hydrogen, methyl, ethyl or phenyl. Further, the present invention also relates to a process for preparation of an intermediate used in the preparation of the compound of formula (I) above.
2. Background art
The compounds, of formula (I), as defined above, are the known compound of which the method for preparation has been described in, for example, European Laid-open Patent Publication No. 87,780, Japanese Laid-open Patent Publication No. (sho) 61-210,003, Japanese Laid-open Patent Publication No. (sho) 61-210,084, etc. Specifically, Japanese Laid-open Patent Publication No. (sho) 61-210,084 describes a method for preparation of the compound (I') comprising the procedure of three steps starting from diethylethoxy methylene malonate according to the following reaction scheme: ##STR3##
According to the above method, in detail, diethylethoxy methylene malonate is initially cyclized with methyl hydrazine to produce the compound of the above formula (III'), a hydroxy group of the compound (III') is subsequently replaced with a chlorine atom by reacting the compound (III') with phosphorus oxychloride (POCl.sub.3) at the temperature of 90.degree. to 100.degree. C. for 65 hours or more and then the resulting compound (II') is reacted with sodium sulfide (Na.sub.2 S) in the presence of dimethylformamide (DMF) solvent at the temperature of 70.degree. to 80.degree. C. for 3.5 hours to prepare the final compound of formula (I').
However, the prior known method as described above has some disadvantages in that the desired compound is prepared through a complex reaction procedure comprising three reaction steps, the product from the first cyclization step may contain a regioisomeric compound of the following formula (I") as by-product to reduce the purity of the product and therefore should be further subjected to a purification procedure, such as recrystallization, and the whole reaction procedure requires a high reaction temperature and a long reaction time and provides the yield as low as 80% and less (particularly, the yield in the chlorination step is merely 30 to 40%): ##STR4##
Thus, the present inventors have continuously studied to develop a method which can more efficiently and economically prepare the compound of the above formula (I) . As a result, we have found that the compound of formula (I) can be efficiently prepared starting from a certain ethylidene dithiocarbazic acid derivative in one step procedure and then completed the present invention.
Therefore, it is an object of the present invention to provide a novel and improved process for preparation of 5-pyrazolemercaptan derivatives having the general formula (I), as defined above.
It is further object of the present invention to provide a novel intermediate to be used in the process for preparation of the compound of formula (I).
Further, it is another object of the present invention to provide a novel process for preparation of some intermediates used for preparing the compound of formula (I).
The more pertinent and important features of the present invention have been outlined above in order that the detailed description of the invention which follows will be better understood and that the present contribution to the art can be fully appreciated. Those skilled in the art can appreciate that the conception and the specific embodiment disclosed herein may be readily utilized as a bases for modifying or designing other structures for carrying out the same purposes of the present invention. Further, those skilled in the art can realize that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the claims.